Series-resonant power converters (SRC) are generally well known. Some known SRCs utilize resonant components to minimize losses in the converter switching devices, e.g., transistors. A typical SRC includes an inverter implemented, for example, using four (4) insulated gate bipolar transistors (IGBTs). The SRC also includes a series coupled resonant inductor and capacitor, a transformer, and a diode bridge having its output coupled to a load. A controller controls the switching, or gating, of the IGBTs.
In operation, the current in the resonant inductor and capacitor resonates with a sinusoidal waveform at a frequency f where f=1/2.pi.(LC).sup.1/2. As a result of this sinusoidal waveform, each IGBT changes its state, or "switches", when the current through the IGBT is near or at zero, thereby minimizing switching losses.
SRCs are utilized, for example, for supplying power to the amplifiers that control gradient magnetic field coils (or gradient coils) of a magnetic resonance (MR) imaging machine. With such MR machines, and in three-dimensional imaging, three (3) gradient coils are utilized to create time-varying gradient magnetic fields in the x, y and z dimensions. Power is supplied by the SRC to each of the gradient amplifiers. Of course, as operating conditions vary, the load demands of each amplifier also vary. Therefore, the SRC must be responsive to a variety of conditions including both low power and high power demands.
With at least one known SRC controller, the controller is implemented as a two-stage scheme to control the output power of the SRC over a wide range of loads. For example, at low output power, the controller utilizes phase modulation to control switching of the IGBTs. At high output power, the controller utilizes frequency modulation to control the IGBT switching frequency. Although such a two-phase scheme is useful for controlling the output power of the SRC, it would be desirable to provide a controller for an SRC which even more quickly responds to load changes and is more efficient. In addition, it would be desirable to improve electromagnetic interference (EMI) performance over a wide range of loads.